Dominant Polarization Mechanism Research Articles

Evolution and mechanism for the terahertz dielectric spectrum of coal during oxidation

In this paper, the dielectric spectrums of lignite during oxidation and its corresponding dielectric response mechanisms were investigated by means of THz-TDS and synchronous thermal analyzer, combined with the Clausius-Mossotti equation and chemical reaction pathway. Results indicate that dipole polarization is the dominant polarization mechanism during coal oxidation in the THz field. There are four dielectric response mechanisms, which gradually participate in the competition of polarity changes with the increase of oxidation temperature. The dielectric response of lignite shows significant temperature dependence. The dielectric constant first decreases until 150 °C due to the dewatering. From 150 °C to 275 °C, the dielectric constant tends to increase. Before 225 °C, the increase is due to the formation of more oxygen-containing bonds. Beyond 225 °C, the coupling effect of the formation of oxygen-containing bonds and the twisting deformation of the molecular chain skeleton become the main reason. Then, due to the fracture of polar bonds, the dielectric constant drops significantly until 400 °C. Beyond 400 °C, the dielectric constant increases, possibly due to enhanced electron migration. The outcomings of the current study help to identify the oxidation stage of coal and enrich the application basis of THz technology in the fields of coal fire identification and mineral processing.

First-principles calculations on ferroelectricity and lattice dynamics of Type-II multiferroic SmMn2O5

In this work, we report on the structural, electronic, and ferroelectric properties of SmMn2O5 by using first-principles density functional theory plus on-site Coulomb interaction (DFT + U) calculations. A thorough analysis was preformed to reveal the competing characteristics of different high-temperature (T) phases and the polarization mechanism in the low-T multiferroic phase. We show that the structural characteristics of the high-T phases have a strong influence on the low-T multiferroicity. In addition to the spin-induced lattice distortion that reduces substantially the purely electronic ferroelectricity, the dominant polarization mechanism in low-T SmMn2O5 still originates from the electronic polarization. By performing mode decomposition of the Hellmann–Feynman forces and the lattice distortion induced by the q = (0.5, 0, 0) magnetic order, we find that the Raman-active Ag mode characterized by the Mn4+O6 octahedron distortion and synergistic displacement of Mn3+ and Sm ions is of primary importance, while the infrared (IR)-active B2u mode plays a secondary role. These findings provide a theoretical foundation for future studies concerning the enhanced magnetoelectric effects of SmMn2O5 due to its pure exchange–striction mechanism.

Magnetically Regulated Disk Formation in the Inner 100 au Region of the Class 0 Young Stellar Object OMC-3/MMS 6 Resolved by JVLA and ALMA

Abstract We have carried out polarization calibration for archival Jansky Very Large Array (JVLA) (∼9 mm) full polarization observations toward the Class 0 young stellar object (YSO) OMC-3/MMS 6 (also known as HOPS-87), and then compared the results with the archival Atacama Large Millimeter Array (ALMA) 1.2 mm observations. The resolved spectral indices show that the innermost ∼100 au region of OMC-3/MMS 6 is marginally optically thin (e.g., τ ≲ 1) at ∼9 mm wavelength, such that the JVLA observations can directly probe the linearly polarized emission from nonspherical dust. Assuming that the projected long axis of dust grains is aligned perpendicular to magnetic field (B-field) lines, we propose that the overall B-field topology resembles an hourglass shape. The geometry of this system is consistent with a magnetically regulated dense (pseudo)disk, although this “hourglass” appears to be ∼40° inclined with respect to the previously reported outflow axis. In contrast, the inner ∼100 au region of this YSO is likely very optically thick (e.g., τ ≫ 1) at ∼1 mm wavelength. The electric field position angles resolved by JVLA and ALMA present ∼90° offsets on this region, which indicate that the dominant polarization mechanism at 1 mm wavelength is dichroic extinction. This is the second case where the (sub)millimeter dichroic extinction is demonstrated by the direct comparison between the JVLA and ALMA polarization observations.

Spectropolarimetry of Seyfert 1 galaxies with equatorial scattering: black hole masses and broad-line region characteristics

Here we present the spectropolarimetric observations of a sample of 30 Type 1 AGNs and an analysis of the observed polarization in these AGNs. The observations have been performed with the 6-meter telescope of SAO RAS using the modified SCORPIO-2 spectropolarimeter. We measured the Stokes parameters for the continuum and the broad H$\alpha$ line and obtained the values of polarization degree and the angle of polarization. We found that equatorial scattering is dominant polarization mechanism in the sample, that allows us to use the observed polarization in the broad lines for determination of the central black hole (BH) masses and characteristics (the inclination and emissivity) of the Broad Line Region (BLR). We demonstrated that the recently proposed method of \cite{ap15} for BH mass measurement gives accurate BH masses which are in a good correlation with the stellar velocity dispersion, and consequently the masses determined by the polarization method can be used with calibration purposes. Additionally we found that the BLR in the sample of 30 AGN has an averaged inclination of $35^\circ\pm9^\circ$ (mostly between 20 and 40 degrees) and emissivity $\alpha\sim -0.57$ that is more flat than one expected for the classical accretion disc $\alpha\sim -0.75$.

AGN black hole mass estimates using polarization in broad emission lines

Context.The innermost regions in active galactic nuclei (AGNs) have not yet been spatially resolved, but spectropolarimetry can provide insight into their hidden physics and geometry. From spectropolarimetric observations in broad emission lines and assuming equatorial scattering as a dominant polarization mechanism, it is possible to estimate the mass of supermassive black holes (SMBHs) residing at the center of AGNs.Aims.We explore the possibilities and limits, and put constraints on the method for determining SMBH masses using polarization in broad emission lines by providing more in-depth theoretical modeling.Methods.We used the Monte Carlo radiative transfer codeSTOKESto explore polarization properties of Type-1 AGNs. We modeled equatorial scattering using flared-disk geometry for a set of different SMBH masses assuming Thomson scattering. In addition to the Keplerian motion, which is assumed to be dominant in the broad-line region (BLR), we also considered cases of additional radial inflows and vertical outflows.Results.We modeled the profiles of polarization plane position angleφ, degree of polarization, and total unpolarized lines for different BLR geometries and different SMBH masses. Our model confirms that the method can be widely used for Type-1 AGNs when viewing inclinations are between 25° and 45°. We show that the distance between the BLR and scattering region (SR) has a significant impact on the mass estimates and the best mass estimates are when the SR is situated at a distance 1.5–2.5 times larger than the outer BLR radius.Conclusions.Our models show that if Keplerian motion can be traced through the polarized line profile, then the direct estimation of the mass of the SMBH can be performed. When radial inflows or vertical outflows are present in the BLR, this method can still be applied if velocities of the inflow/outflow are less than 500 km s−1. We also find that models for NGC 4051, NGC 4151, 3C 273, and PG0844+349 are in good agreement with observations.

Infrared polarimetry of Mrk 231: scattering off hot dust grains in the central core

We present high-angular (0.17$-$0.35 arcsec) resolution imaging polarimetric observations of Mrk 231 in the 3.1 $\mu$m filter using MMT-Pol on the 6.5-m MMT, and in the 8.7 $\mu$m, 10.3 $\mu$m, and 11.6 $\mu$m filters using CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already published observations, we compile the 1$-$12 $\mu$m total and polarized nuclear spectral energy distribution (SED). The total flux SED in the central 400 pc is explained as the combination of 1) a hot (731 $\pm$ 4 K) dusty structure, directly irradiated by the central engine, which is at 1.6 $\pm$ 0.1 pc away and attributed to be in the pc-scale polar region, 2) an optically-thick, smooth and disk-like dusty structure (`torus') with an inclination of 48 $\pm$ 23$^{\circ}$ surrounding the central engine, and 3) an extinguished (A$_{\mbox{V}} =$ 36 $\pm$ 5 mag) starburst component. The polarized SED decreases from 0.77 $\pm$ 0.14 per cent at 1.2 $\mu$m to 0.31 $\pm$ 0.15 per cent at 11.6 $\mu$m and follows a power-law function, $\lambda^{\sim0.57}$. The polarization angle remains constant ($\sim$108$^{\circ}$) in the 1$-$12 $\mu$m wavelength range. The dominant polarization mechanism is explained as scattering off hot dust grains in the pc-scale polar regions.

Impedance spectroscopy and mechanical response of porous nanophase hydroxyapatite–barium titanate composite

The present study aims to develop the porous nanophase hydroxyapatite (HA)–barium titanate (BT) composite with reasonable mechanical and electrical properties as an electrically-active prosthetic orthopedic implant alternate. The porous samples (densification ~40–70%) with varying amounts of BT (0, 25, 35 and 100vol.%) in HA were synthesized using optimal spark plasma sintering conditions, which revealed the thermochemical stability between both the phases. The reasonably good combination of functional properties such as compressive [(236.00±44.90)MPa] and flexural [(56.18±5.82) MPa] strengths, AC conductivity [7.62×10−9(ohm-cm)−1 at 10kHz] and relative permittivity [15.20 at 10kHz] have been achieved with nanostructured HA-25vol.% BT composite as far as significant sample porosity (~30%) is concerned. Detailed impedance spectroscopic analysis was performed to reveal the electrical microstructure of developed porous samples. The resistance and capacitance values (at 500°C) of grain (RG, CG) and grain boundary (RGB, CGB) for the porous HA-25vol.% BT composite are (1.3×107 ohm, 3.1×10−11F) and (1.6×107 ohm, 5.9×10−10F), respectively. Almost similar value of activation energy (~1–1.5eV) for grain and grain boundary has been observed for all the samples. The mechanism of conduction is found to be same for porous monolithic HA as well as composite samples. Relaxation spectroscopic analyses suggest that both the localized as well as long range charge carrier translocations are responsible for conduction in these samples. The degree of polarization of porous samples has been assessed by measuring thermally stimulated depolarization current of the poled samples. The depolarization current is observed to depend on the heating rate. The maximum current density, measured for HA-25vol.% BT sample at a heating rate of 1°C/min is 2.7nA/cm2. Formation of oxygen vacancies due to the reduced atmosphere sintering contribute to the space charge polarization, which is obtained as the dominant polarization mechanism in the developed porous samples. Overall, such integrated functional responses do establish spark plasma sintered porous HA–BaTiO3 nanocomposite as potential alternative for electroactive prosthetic orthopedic implants.

Space charge polarization induced augmented in vitro bioactivity of piezoelectric (Na,K) NbO3

The present study reports the influence of chemical and electrical treatments (CET) of Li-modified sodium potassium niobate [Li0.06(Na0.5K0.5)0.94NbO3, LNKN] piezo-biomaterial on in vitro bioactivity. The chemical treatment of LNKN substrate was performed by Ti sputtering, followed by immersion in NaOH aqueous solution for various time durations and subsequently, the heat treatment. The chemically treated LNKN substrates were then corona poled. The in vitro biomineralization study suggested that the integrated effect of chemical as well as electrical treatments of LNKN piezoceramics stimulates the early stage apatite crystallization as compared to that of the mere chemical treatment. To reveal the mechanism of polarization on crystal formation, the thermally stimulated depolarization currents (TSDC) were measured before and after the chemically as well as electrically treated substrates being soaked in simulated body fluid for various time durations. It has been observed that the release of surface charge in the low temperature region (∼60 °C) has significant effect on apatite formation and growth. The surface charge density decreases with increase in the soaking period and hence, the influence of surface charge on apatite crystallization can be realized. The dielectric relaxation in the low frequency region as well as TSDC analysis suggests that the space charge polarization is dominant polarization mechanism in the CET piezo-biomaterial.

Dielectrophoresis force spectroscopy for colloidal clusters

Optical trapping-based force spectroscopy was used to measure the frequency-dependent DEP forces and DEP crossover frequencies of colloidal polymethyl methacrylate spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where alternating current elecroosmosis flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other hand, the power-law dependence of the crossover frequency on the particle radius with an exponent of -2 indicates the dielectric relaxation is more likely because of ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.

Charge compensation, electrical and dielectric behavior of lanthanum doped CaCu3Ti4O12

Mechanism of charge compensation on lanthanum, (La3+) substitution on Ca site in calcium copper titanate (CaCu3Ti4O12), and its effect on resulting electrical and dielectric properties has been studied in the present investigation. For this purpose samples were prepared according to two stoichiometries viz. LaxCa(1−3x/2)Cu3Ti4O12 (x≤0.09) and LaxCa(1−x)Cu3Ti4O12 (x=0.03) by solid state ceramic route. The former represents ionic compensation while the later is in accordance with electronic compensation. Nature of charge carriers is identified by measuring Seebeck coefficient which is found to be negative in the entire range of measurement. In order to understand the mechanism of conduction, ac conductivity is measured as a function of temperature and frequency. Space charge polarization is the dominant polarization mechanism phenomenon at low frequency and high temperature while orientation polarization dominates at low temperature and high frequency. Impedance analysis confirms the formation of internal barrier layers which is responsible for high dielectric constant in these samples.

Electric-field-induced polarization and interactions of uncharged colloids in salt solutions

The electric-field-induced charge distribution and potential around a colloidal sphere and rod in salt solutions are analyzed. The resulting field-induced colloid-colloid interactions are calculated for specific orientations. The colloids are assumed to be uncharged (or very weakly charged), such that the deflection of ion fluxes by the cores of the colloids is the dominant polarization mechanism (which is referred to as volume-polarization). Explicit expressions are derived for the frequency-dependent charge distribution and the potential in case of a symmetric electrolyte. It is shown that colloid-colloid interactions due to the induced charge distributions can be much larger than the thermal energy, and are therefore sufficiently strong to give rise to electric-field-induced phase transitions. The present study is a first step towards a quantitative description of field-induced transitions for systems where volume-polarization is the dominant polarization mechanism.

Candidate Type II Quasars from the Sloan Digital Sky Survey. III. Spectropolarimetry Reveals Hidden Type I Nuclei

We have conducted spectropolarimetry of 12 type II (obscured) quasar candidates selected from the spectroscopic database of the Sloan Digital Sky Survey on the basis of their emission-line properties. Polarization was detected in all objects, with nine being highly polarized (>3%) and with polarization reaching as high as 17% in two objects. Broad lines were detected in the polarized spectra of five objects. These observations prove beyond a reasonable doubt that the objects in our sample are indeed type II quasars in that they harbor luminous UV-excess active galactic nuclei (AGNs) in their centers and that the direct view to the AGN is highly obscured. For three of the objects in this paper, we have obtained Hubble Space Telescope (HST) images in three bands. The HST observations combined with the spectropolarimetry data imply that scattering off material outside the obscuration plane is the dominant polarization mechanism. In all three objects the sizes of scattering regions are a few kiloparsecs. For one object, the extent of the scattering region coupled with the characteristics of the polarized spectrum argues strongly that dust scattering rather than electron scattering dominates the polarized light. Our observations are well described by the basic orientation-based unification model of toroidal obscuration and off-plane scattering, implying that the model can be extended to include at least some high-luminosity AGNs.

Scattering and Dichroic Extinction: Polarimetric Signatures of Galaxies

Radiation transfer models of the polarization structure of galaxies are presented. These models include dust scattering and dichroic extinction (the mechanism believed to be responsible for producing the observed interstellar polarization within our own Galaxy). As with previous efforts to model galactic polarization maps, the scattering-only models produce a centrosymmetric pattern of the polarization vectors when one is viewing a galaxy pole-on and vectors perpendicular to the galactic plane for highly inclined galaxies. In general, polarization observations of galaxies show vectors that are parallel to the galactic plane or that follow the spiral arm structure, which indicates that the dominant polarization mechanism is from the transmission of light through aligned grains (dichroic extinction). When we include a toroidal magnetic field to align the dust grains and the resulting dichroic polarization in our radiation transfer simulations, we do indeed find that the polarization is parallel to the galactic disk. Depending on the optical depth of the galaxy and the dust properties (albedo, phase function, peak scattering polarization), the polarization patterns at different regions within the galaxy show the signatures of dichroism, scattering, and null points owing to cancellation between the two polarizing mechanisms. We find that in order to reproduce the polarization structure of external galaxies, it is essential to include dichroic extinction in the radiation transfer.

Optical polarization in distant radio galaxies

We review the optical polarization properties of radio galaxies with redshifts greater than 0.1. We find a strong tendency for the E-vector to be either perpendicular or parallel to the radio axis, and perpendicular to the axis in all the high-redshift galaxies. The observed degree of linear polarization is correlated with redshift and radio power, and is anticorrelated with the rest-frame wavelength of the measurement and the radio spectral index. No correlation is found with absolute magnitude. For the high-polarization objects (P ≥ 3 per cent), the polarization also increases with the radio asymmetry parameter Q, which is thought to be related to the interstellar-medium density ratio. After correcting the polarization for stellar dilution, only weak correlations with redshift and rest-frame wavelength remain, suggesting that the redshift dependence could be entirely explained as a wavelength-dependent contrast effect. It is possible to divide the radio galaxies into two groups: the high-redshift objects (z ≥ 0.6) with high perpendicular polarization and strong radio-optical alignments, and the low-redshift sources with low polarization having either parallel or perpendicular E-vectors. The dominant polarization mechanism for the first group must be the scattering of nuclear radiation. We present a dust-scattering model that explains the structure, the polarization properties and the spectral energy distribution of the ultraviolet aligned light with optically thin Mie scattering of quasar radiation emitted in a cone of 47° half-opening angle. The required amount of spherically distributed dust is |$(1-3) \times {10}^{8} {\text M}_{\odot}$| (consistent with the IRAS data), which produces only a small dust extinction (Av ~ 0.1) along a line of sight to a quasar. The low-redshift radio galaxies do not have a single dominant polarization mechanism. In addition to scattering, polarization by transmission through aligned dust grains and synchrotron radiation can play a role in some of these objects.

A LABORATORY INVESTIGATION OF ELECTRICAL ANISOTROPY IN PRECAMBRIAN ROCKS

Laboratory electrical anisotropy measurements were made on selected dry metasediments and metavolcanics from the Precambrian complex of northern Michigan. Directional ac conductivity and dielectric‐constant values were obtained in six or more directions for each sample. These directional values were used to obtain a least‐square determination of the six independent coefficients needed to completely define the symmetric second‐rank, conductivity and dielectric constant tensors. Tensor principal values and directions were obtained from these coefficients. The results of this investigation indicate that metamorphic rocks may be characterized by strongly anisotropic electrical properties. The tensor representation surface symmetries reflect the symmetry of the rock fabric. Electrical anisotropy tends to increase, and the symmetry of the representation surfaces tends to decrease, at lower signal frequencies. The frequency spectra for all samples follow a relaxation‐type model, with critical frequencies occurring between 200 and 600 hz, indicating interfacial polarization as the dominant polarization mechanism.

Theory of Spin Effects in Photoionization: The(ns)2System

The theory of the spin polarization of photoelectrons and of its correlation with angle of ejection is extended by the derivation of a cross section for photoionization of a ${(\mathrm{ns})}^{2}^{1}S$ atom. The effect of an isolated resonance in the absorption continuum is treated by Fano's theory of configuration interaction. The connection between photoionization and electron scattering leads to the use of complex ingoing-wave eigenfunctions and to an expression of the photoionization cross section in terms of ${\stackrel{^}{S}}^{\frac{1}{2}}$, the square root of the scattering matrix. The spin polarization is influenced by the exclusion principle and by the action of the spin-orbit forces in the discrete and continuum states. Configuration interaction between discrete and continuum is shown to be the dominant polarization mechanism near an autoionization line and can yield a high degree of polarization. The mean polarization vector has components in and out of the plane of photoemission, and its direction varies rapidly as the light frequency varies across a resonance. Two intensity vectors are introduced, derivable, respectively, from the experimental total and differential cross sections for photoionization. The rotation matrix connecting the two intensity vectors is expressed in terms of the interaction parameters which determine ${\stackrel{^}{S}}^{\frac{1}{2}}$.